xni 



PHYLUM CHORDATA 



477 



The mammary glands, by the secretion of which the young are 

 nourished, are specially developed cutaneous glands. In the 

 Prototheria they differ somewhat widely from those of the rest of 

 the Mammalia in structure, and they also differ in the absence of 

 teats. They consist of two groups of very large tubular follicles, the 

 ducts of which open on the ventral surface. In Echidna (Fig. 

 1114) the two areas on which the ducts open become depressed 

 towards the breeding season to give rise to a pair of pouches 

 the mammary pouches. A large brood-pouch or marsupium is 

 subsequently formed, and the egg is deposited in this. When the 

 A B 



- in. 



d. 



FIG. 1114. Echidna aculeata. A, lower surface of brooding female ; B, dissection showing 

 a dorsal view of the marsupium and mammary glands ; t t, the two tufts of hair projecting 

 from the mammary pouches from which the secretion flows ; b.m. brood-pouch or marsu- 

 pium ; cl. cloaca ; g. m. groups of mammary glands. (From Wiedersheim's Comparative 

 Anatomy, after W. Haacke.) 



young animal is hatched it is sheltered in the posterior deeper 

 part of this marsupium, while in the shallower anterior part lie 

 the mammary pouches. In Ornithorhynchus mammary pouches 

 are indicated only by extremely shallow depressions, and no 

 marsupium is developed. 



In the higher Mammals, when the mammary glands are first 

 developed (Fig. 1115), a depression (mammary pouch) is formed, 

 from the floor of which branching cylindrical strands of epidermis 

 grow inwards to give rise to the glands. At a later stage there 

 is developed around the opening or openings of the mammary 



'. HI. 



